The present invention relates generally to analytical instruments, and specifically to instruments and methods for thin film analysis using X-rays.
X-ray reflectometry (XRR) is a well-known technique for measuring the thickness, density and surface quality of thin film layers deposited on a substrate. Such measurements are particularly useful in evaluating layers deposited on semiconductor wafer substrates in the course of integrated circuit manufacture.
X-ray reflectometers are sold by a number of companies, among them Technos (Osaka, Japan), Siemens (Munich, Germany) and Bede Scientific Instrument (Durham, UK). Such reflectometers typically operate by irradiating a sample with a beam of X-rays at grazing incidence, i.e., at a small angle relative to the surface of the sample, near the total external reflection angle of the sample material. Measurement of X-ray intensity reflected from the sample as a function of angle gives a pattern of interference fringes, which is analyzed to determine the properties of the film layers responsible for creating the fringe pattern. The X-ray intensity measurements are commonly made using a detector mounted on a goniometer. More recently, fast X-ray reflectometers have been developed using position-sensitive detectors, such as a proportional counter or an array detector, typically a photodiode array or charge-coupled device (CCD).
For example, U.S. Pat. No. 5,619,548, to Koppel, whose disclosure is incorporated herein by reference, describes an X-ray thickness gauge based on reflectometric measurement. A curved, reflective X-ray monochromator is used to focus X-rays onto the surface of a sample. A position-sensitive detector, such as a photodiode detector array, senses the X-rays reflected from the surface and produces an intensity signal as a function of reflection angle. The angle-dependent signal is analyzed to determine properties of the structure of a thin film layer on the sample, including thickness, density and surface roughness.
U.S. Pat. No. 5,923,720, to Barton et al., whose disclosure is incorporated herein by reference, also describes an X-ray spectrometer based on a curved crystal monochromator. The monochromator has the shape of a tapered logarithmic spiral, which is described as achieving a finer focal spot on a sample surface than prior art monochromators. X-rays reflected or diffracted from the sample surface are received by a position-sensitive detector.
U.S. Pat. No. 5,740,226, to Komiya et al., describes a method for analyzing X-ray reflectometric data to determine film thickness. After measuring X-ray reflectance as a function of angle, an average reflectance curve is fitted to the fringe spectrum. The average curve is based on a formula that expresses attenuation, background and surface roughness of the film. The fitted average reflectance curve is then used in extracting the oscillatory component of the fringe spectrum. This component is Fourier transformed to find the film thickness.
In order to obtain accurate measurements of film thickness, it is necessary to precisely calibrate the angular scale of the reflection. Such a calibration requires, inter alia, exact control of the zero angle of reflection, so that the angle of the reflected beam relative to the surface can be determined accurately. (In the context of the present patent application and in the claims, the term xe2x80x9czero anglexe2x80x9d refers to the orientation of a tangent to the reflecting surface at the point of incidence of the radiation.) To make reflectometric measurements with optimal accuracy, the zero angle at the measurement point should be known to within 0.005xc2x0.
Although semiconductor wafers appear to be flat, in practice wafers typically deform slightly when held by a vacuum chuck during production or inspection. The deformation is due both to the vacuum force exerted by the chuck and to the weight of the wafer itself. Furthermore, the chuck may have imperfections, such as a slight bend in its axis, that cause deviations in the zero angle of the wafer as it rotates. As a result, inclination of the surface at different sample points on the surface of a wafer may vary by as much as 0.1-0.2xc2x0. Therefore, to perform accurate reflectometric measurements at a well-defined measurement point, it becomes necessary to recalibrate the zero angle at each point that is tested on the wafer surface.
It is an object of some aspects of the present invention to provide improved methods and systems for reflectometry.
It is a further object of some aspects of the present invention to provide methods and devices that enable rapid, accurate determination of the zero angle of a surface under reflectometric inspection.
In preferred embodiments of the present invention, the zero angle of a surface under inspection is calibrated by measuring reflections of X-ray beams from the surface at two different, known wavelengths, xcex1 and xcex2. The beams are aligned so as to impinge upon the surface at the same point and along substantially the same direction. Each of the beams generates a reflectometric fringe pattern, which allows the critical angle for total external reflection from the surface to be observed at each of the two wavelengths. Even when the precise zero angle of the surface is not known, the difference between the critical angles at the two different wavelengths can be measured with high precision.
In accordance with known physical principles, the critical angle at X-ray wavelengths is equal to a constant, k, which depends on the density of the reflecting surface layer, multiplied by the wavelength itself. The precise measurement of the difference in the critical angles at the two different measurement wavelengths can thus be used to accurately calculate k with respect to the surface under inspection. Once k is known, the zero angle of the surface at the measurement point is calibrated simply by subtracting kxcex from the observed critical angle at either of the known wavelengths. The pattern of reflected fringes at either or both of xcex1 and xcex2 can then be analyzed to accurately determine local surface properties including thickness, density and roughness of think film layers on the surface.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method for testing a surface, including:
finding respective first and second critical angles for total external reflection of radiation from an area of the surface at first and second wavelengths; and
comparing the first and second critical angles to determine an orientation of a tangent to the surface in the area.
Preferably, comparing the first and second critical angles includes taking an angular difference between the first and second critical angles, and calculating, based on the angular difference, a property of the surface for use in determining the orientation of the tangent. Most preferably, calculating the property includes finding a constant k such that the angular difference between the first and second critical angles is substantially equal to |k(xcex2-xcex1)|, wherein xcex1 and xcex2 are the first and second wavelengths, respectively, and setting kxcex1, equal to the first critical angle so as to find the orientation of the tangent.
Preferably, finding the first and second critical angles includes irradiating the surface with first and second beams of the radiation at the first and second wavelengths, respectively, wherein the first and second beams both impinge on the surface in the area along substantially the same direction. Further preferably, finding the first and second critical angles includes detecting the radiation reflected from the surface using a common detector for the first and second beams. Most preferably, detecting the radiation includes detecting the radiation at the second wavelength while preventing the first beam from impinging on the surface. Alternatively, since the first and second beams have respective first and second photon energies dependent on the first and second wavelengths, detecting the radiation includes discriminating between the radiation detected at the first and second wavelengths responsive to the respective photon energies.
In a preferred embodiment, irradiating the surface includes generating the first and second beams using first and second radiation sources, respectively. Preferably, irradiating the surface includes focusing the first and second beams onto the surface using first and second crystal monochromators, respectively, in mutually-adjacent positions.
In another preferred embodiment, irradiating the surface includes generating the first and second beams using a single radiation source that emits the radiation at the first and second wavelengths. Preferably, irradiating the surface includes focusing the first and second beams onto the surface using a single crystal monochromator for both the first and second wavelengths. Most preferably, the second wavelength is approximately equal to half the first wavelength, so that the crystal monochromator diffracts a first order of the first beam and a second order of the second beam toward the area of the surface. Alternatively, the crystal monochromator includes first and second crystal elements having respective first and second crystal spacings, selected so that the first crystal element diffracts the first beam toward the area of the surface, while the second crystal element diffracts the second beam toward the area of the surface.
Preferably, finding the critical angles includes detecting an oscillatory pattern in the radiation reflected from the area as a function of elevation angle relative to the surface, and the method includes analyzing the pattern, responsive to the orientation of the tangent, so as to determine a property of the surface. In a preferred embodiment, the surface has at least one thin film layer formed thereon, and finding the critical angles includes irradiating the surface with X-rays at the first and second wavelengths, and analyzing the pattern includes analyzing the X-rays reflected from the surface to determine the property of the at least one thin film layer. In a further preferred embodiment, detecting the oscillatory pattern includes observing the oscillatory pattern at both of the first and second wavelengths.
There is also provided, in accordance with a preferred embodiment of the present invention, apparatus for testing a surface, including:
a radiation source, adapted to irradiate an area of the surface at first and second wavelengths;
a detector, adapted to receive radiation reflected from the surface and to generate a signal responsive thereto; and
a signal processor, coupled to receive and analyze the signal so as to find respective first and second critical angles for total external reflection of radiation from an area of the surface at the first and second wavelengths, and to compare the first and second critical angles to determine an orientation of a tangent to the surface in the area.
Preferably, the radiation source is adapted to irradiate the surface with first and second beams of the radiation at the first and second wavelengths, respectively, so that the first and second beams both impinge on the surface in the area along substantially the same direction. Further preferably, the detector has a shape and size chosen so as to detect the radiation reflected from the surface in both the first and second beams, substantially without movement of the detector. Most preferably, the radiation source includes a filter, which is operable to prevent the first beam from impinging on the surface while the detector detects the second beam.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a crystal monochromator, including first and second crystal elements, having respective first and second crystal spacings chosen so that the crystal elements diffract radiation incident thereon at respective first and second wavelengths at a selected Bragg angle, the crystal elements having a curvature chosen so as to focus the radiation at the first and second wavelengths to a common focal area.
Preferably, the first and second crystal elements include first and second crystals having respective front surfaces with the chosen curvature, positioned side by side so that the front surfaces define a common curve. Alternatively, the first crystal element includes a bulk crystal having a front surface with the chosen curvature, and the second crystal element includes a thin layer formed on the front surface of the first crystal element.
There is further provided, in accordance with a preferred embodiment of the present invention, a method for testing a surface, including:
finding respective first and second critical angles for total external reflection of radiation from an area of the surface at first and second wavelengths; and
comparing the first and second critical angles to determine a property of the surface.
There is moreover provided, in accordance with a preferred embodiment of the present invention, apparatus for testing a surface, including:
a radiation source, adapted to irradiate an area of the surface at first and second wavelengths;
a detector, adapted to receive radiation reflected from the surface and to generate a signal responsive thereto; and
a signal processor, coupled to receive and analyze the signal so as to find respective first and second critical angles for total external reflection of radiation from an area of the surface at the first and second wavelengths, and to compare the first and second critical angles to determine a property of the surface.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: